An additional constant multiplicative term was incorporated in the models, utilizing the CONSTANT model inbuilt in XSPEC, to adjust the factors related to the cross-instrument calibration uncertainties. This constant is kept fixed at 1 for Swift/XRT, and the spectra were fitted by keeping it free for FPMA and FPMB. In this way, the best-fit constants for FPMA and FPMB represent the relative cross-instrument calibration factors with respect to Swift/XRT. This relative cross-instrument factor for E1 was estimated as \(1.16\pm 0.02\)  (\(\sim \) 16%) for both FPMA and FPMB, while for E2 it was found to be \(1.08\pm 0.01\)  (\(\sim \) 8%) for both telescopes. The recommended acceptable range for the cross-instrument calibration factor between Swift and NuSTAR is 3\(-\) 20 % [1]}, [2]}. Hence, our values clearly fall within the acceptable range as recommended in the literature.
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